Ink additives for improved ink-jet performance

ABSTRACT

Oxo anions (both singly and multiply charged), such as phosphates, polyphosphates, and phosphate esters, serve as additives, and in the case of cationic dyes, may serve as replacement counter-ions, for use in thermal ink-jet inks to reduce kogation significantly. The addition of such additive essentially eliminates kogation for the life of ink pens. Further, such additives will prevent kogation for inks containing dyes with negatively-charged water-solubilizing groups, such as sulfonate and carboxylate.

TECHNICAL FIELD

The present invention relates to inks used in ink-jet printers, and,more particularly, to an ink used in thermal ink-jet printers havingimproved kogation properties.

BACKGROUND ART

Thermal ink-jet printers offer a low cost, high quality, andcomparatively noise-free option to other types of printers commonly usedwith computers. Such printers employ a resistor element in a chamberprovided with an egress for ink to enter from a plenum. The plenum isconnected to a reservoir for storing the ink. A plurality of suchresistor elements are arranged in a particular pattern, called aprimitive, in a printhead. Each resistor element is associated with anozzle in a nozzle plate, through which ink is expelled toward a printmedium. The entire assembly of printhead and reservoir comprise anink-jet pen.

In operation, each resistor element is connected via a conductive traceto microprocessor, where current-carrying signals cause one or moreselected elements to heat up. The heating creates a bubble of ink in thechamber, which is expelled through the nozzle toward the print medium.In this way, firing of a plurality of such resistor elements in aparticular order in a given primitive forms alphanumeric characters,performs area-fill, and provides other print capabilities on the medium.

A problem with inks used in such thermal ink-jet printers is that therepeated heating of the resistor element over several hundreds ofthousand or over millions of firings can cause breakdown of the ink,with consequent fouling of the surface of the resistor element. Thisprocess has been termed “kogation”, which is defined as the build-up ofresidue (koga) on the resistor surface. The build-up of residue degradespen performance.

Various ink compositions and processes have been developed in an effortto reduce kogation. For example, in the anionic dyes (sulfonate orcarboxylate) commonly employed in aqueous inks used in thermal ink-jetprinting, sodium is generally the counter-ion used. However, while dyescontaining sodium counter-ions generally provide good print quality,sodium counter-ions have been found to contribute to the kogationproblem.

One solution has been to partially or totally eliminate sodium.Successful replacement counter-ions are lithium and tetramethylammonium.

The need remains for the development of inks having reduced kogation,and hence longer life, using low cost chemicals with minimal additionalprocessing.

DISCLOSURE OF INVENTION

In accordance with the invention, oxo anions serve as additives, and inthe case of cationic dyes, may serve as replacement counter-ions, foruse in thermal ink-jet inks to reduce kogation significantly. The oxoanions of the invention may be singly or multiply charged.

The addition of such additive essentially eliminates kogation for thelife of ink pens, such as those used in Hewlett-Packard's DeskJetprinter. Further, such additives will prevent kogation for inkscontaining dyes with negatively-charged water-solubilizing groups, suchas sulfonate and carboxylate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, on coordinates of weight (in nanograms, ng) and number of cycles(resistor firings), is a plot of drop weight versus cycles for threedifferent energies of an ink not including the additive of theinvention, depicting the effect of kogation on drop weight out to 5million cycles; and

FIG. 2 is a plot similar to that of FIG. 1, but for an ink containingthe additive of the invention, depicting essentially no kogation out to10 million cycles.

BEST MODES FOR CARRYING OUT THE INVENTION

Inks benefitted by the practice of the invention comprise a vehicle anda dye. The vehicle typically comprises one or more water-miscibleorganic compounds, such as a glycol or glycol ether and water. The dyemay be any of the anionic or cationic dyes. The dye is typically presentin an amount ranging from about 1 to 12% (by weight), although more orless dye may be used, depending on the vehicle/dye system, the desiredoptical density, etc. Typically, the dye concentration is about 2 to 6%(by weight). All amounts herein are by weight, unless otherwiseindicated.

Particularly employed as inks herein are ICI dyes 286 and 287,preferably in a 50—50 mixture. However, any of the well-known dyes mayalternately be used.

The particular water-miscible organic compounds and their concentrationdoes not form a part of this invention. However, examples of suchcompounds include glycols such as ethylene glycol, diethylene glycol,propylene glycol, polyethylene glycol, etc., and pyrrolidones, such as2-pyrrolidone. Usually, the glycol is present in an amount up to about50%, and more typically up to about 10%, with the balance water. Thepyrrolidine is usually present in an amount of about 7 to 10%, with thebalance water.

Other additives may be added to the ink, such as fungicides,bactericides, pH adjusters, and the like, as is well-known. Suchadditives, and the materials comprising the vehicle and dye are of apurity commonly found in normal commercial practice.

In accordance with the invention, the addition of a compound containingan oxo anion significantly reduces kogation, and may even eliminate itentirely. Examples of such oxo anions include phosphates (both PO₄ ³⁻and P₂O₇ ⁴⁻) and phosphate esters (both mono-organo, ROPO₃ ²⁻, anddi-organo, (RO)₂PO₂ ⁻). For the phosphate esters, R is an alkyl oraromatic group. The R groups for the di-organo phosphates may be thesame or different. The organic R group can also be substituted withvarious functional groups.) Further examples of oxo anions beneficiallyemployed in the practice of the invention include, in descending orderof preference, arsenate (AsO₄ ³⁻), molybdate (Mo₇O₂₄ ⁶⁻), sulfate (SO₄²⁻), sulfite (SO₃ ²⁻), and oxalate (C₂O₄ ²⁻). Anions other than thesemay not have a beneficial effect. For example, nitrate and thiocyanateanions are ineffective with the ICI dyes mentioned above. As usedherein, an oxo anion is a class of anions in which various elements arebound to oxygen and which bear an overall negative charge in aqueoussolution.

The most effective additive to date are phosphate salts; added either asdibasic (HPO₄ ²⁻) monobasic (H₂PO₄ ⁻), polyphosphates such asdiphosphate (P₂O₇ ⁴⁻), or phosphate esters.

The phosphate species in solution is determined by the pH of the ink. Inthe pH range of 8 to 9 (typical for inks containing ICI dyes), thepredominate species for both mono and dibasic phosphate is HPO₄ ²⁻.

FIG. 1 depicts a measure of kogation from an ink comprising a vehicle of10% 2-pyrrolidone and 0.2% sodium borate, the balance water, and 2.2% ofa 50—50 mixture of ICI 286/287 dyes (NH₄ ⁺ form). The pH was adjusted to8.5 with NH₄OH. In this ink, no additive was used, and it is clear thatthere is a large decrease in drop volume (determination is by weight)with this ink, beginning almost immediately.

For comparison, kogation results for the same base ink composition with0.1 wt% ammonium phosphate are depicted in FIG. 2. The addition ofammonium phosphate clearly yields an ink with constant drop volume. Thisink evidence substantially flat behavior even out to 30 million cycles.

The selection of the counter-ion is not critical, other than it notadversely interfere with the reduction in kogation. Examples of suitablecations include alkali metals, ammonium, and alkyl ammonium. Anespecially efficacious compound is ammonium phosphate. Phosphate ion canalso be added as phosphoric acid (H₃PO₄), along with neutralization withan appropriate base.

The concentration of the kogatin-reducing additive (anions) of theinvention ranges from about 9 mg/L to 14 wt%, based on the oxo anion.Less than about 9 mg/L, while effective, is not enough to yield stabledrop volumes out to several million firings. Greater than about 14 wt%provides no further benefit. Preferably, the concentration of the oxoanion ranges from about 0.01 to 1 wt%.

As indicated earlier, kogation degrades pen performance. A decrease inpen performance can be monitored by measuring (weighing) drops firedfrom a pen. A change in drop volume indicates the formation of resistorresidues.

Without subscribing to any particular theory, it appears that thekogation effect is due to adsorption of dye and/or decompositionproducts of ink on the resistor surface. The appearance and increase inadsorbed dye or decomposition products apparently reduces the volume ofink fired. The additive of the invention is believed to eliminate orreduce the adsorption process.

The addition of ammonium phosphate to inks at relatively lowconcentrations (0.02 to 0.5%) yields inks which have constant dropvolumes out to many million drops. For example, inks containing suchamounts of ammonium phosphate have shown that ink volumes remainconstant to at least 30 million drops. The same inks without ammoniumphosphate do not have constant drop volumes, as indicated earlier withreference to FIGS. 1 and 2.

Examination of the resistors from inks without and with the addition ofammonium phosphate shows that there is a large amount of residue (koga)on resistors for inks without phosphate fired only to 5 million cyclesand essentially clean resistor surfaces for inks containing phosphatewhen fired twice that number of cycles (10 million).

Auger Electron Spectroscopy has determined that the resistor residueprimarily comprises carbon. Other elements, such as nitrogen, oxygen,and sulfur are at relatively lower concentrations. The resistor surfacesexposed to non-phosphate containing inks have a thick carbon layer(corresponding to adsorbed dye and/or ink decomposition products),whereas the resistor surfaces exposed to phosphate-containing inks haveonly a very thin carbon layer, together with some phosphorus.Apparently, phosphate is being adsorbed on the resistor surface, andprevents the adsorption of dye and/or ink decomposition productsthereon.

Interestingly, the foregoing suggests that the process of kogation isreversible. Indeed, pens have been kogated by firing with an ink thatdoes not contain phosphate to several million cycles until the dropvolumes decrease considerably. Refilling these pens with aphosphate-containing ink yields complete recovery of the pen. That is,the drop volumes rise to a “normal level” of about 140 pL.

The pH of the inks is adjusted to be within the range of about 3 to 10,and preferably about 8 to 9 for the ICI dye examples discussed herein,using commonly-employed pH adjusters.

INDUSTRIAL APPLICABILITY

The oxo anion additive of the invention is expected to find use in inksused in thermal ink-jet printers.

EXAMPLES Example 1

In this example, the preparation of an ink containing phosphate isdescribed.

The ink comprised a vehicle of 10% 2-pyrrolidone, 0.2% sodium borate asa pH buffer, and the balance deionized water and 2.2% of 50—50 ICI287/287 dye. Monobasic ammonium phosphate (NH₄H₂PO₄) was added to theforegoing ink to provide a concentration of 0.1 wt% therein. The initialpH was adjusted to 8.5 with concentrated NH₄OH.

Example 2

The ink from Example 1 was tested for kogation out to 10 million cycles.The parameter which has been used to evaluate the effectiveness of anadditive is drop volume. In this test, droplets ejected from a pen arecollected and weighed in a pan on an analytical balance. An averageweight is obtained and is commonly referred to as drop volume inpicoliters (pL). The current test operates at three different energies(15%, 30%, and 45%) over (OE) that required to fire a droplet from anozzle in order to obtain a range of performance. In a given printer anda given pen under normal operating conditions, a pen will be operatingat a single energy.

The results are depicted in FIG. 2, as discussed above. In FIG. 2 (andin FIG. 1), the dashed line represents 15% OE, the heavy solid linerepresents 30% OE, and the light solid line represents 45% OE.

Example 3

For comparison, the same ink as in Example 1 was prepared, but omittingthe ammonium phosphate. The ink was tested as in Example 2. The resultsare depicted in FIG. 1, as discussed above.

Clearly, the ink without ammonium phosphate is seen to exhibit kogationwithin a very short number of cycles, while the ink containing ammoniumphosphate is stable against kogation out to at least 10 million cycles.

Example 4

A mixture of dimethyl phosphate (55%) and monomethyl phosphate (45%) wasadded to an ink which was the same as in Example 3, except that theamount of pyrrolidone in the vehicle was 7.5%. The total phosphate esterconcentration was 0.5 wt% and the pH of the ink was adjusted to 8.5.This ink had stable drop volumes when tested to 4.8 million cycles.

Example 5

Addition of 0.2% ammonium phosphate to an ink prepared with the sodiumform of Direct Black 168 (1.9 wt%) in a vehicle of 5.5% diethyleneglycol, balance water, yielded excellent results. The drop volumes wereconstant when tested to 9 million cycles.

Example 6

Addition of 0.5% of a mixture of dimethyl phosphate (55%) and monomethylphosphate (45%) to an ink prepared with the lithium form of Acid Red 27(3 wt%), a magenta dye, in a vehicle of 5.5 wt% diethylene glycol,balance water, yielded stable drop volumes for all energies when testedto 4.8 million cycles.

Example 7

Addition of 0.2% ammonium phosphate to an ink prepared with Acid Red 27(3 wt%) in a vehicle of 5.5 wt% diethylene glycol, balance water,yielded stable drop volumes for all energies for all energies (15%, 30%,45% OE) after an initial rise, which converged at approximately 160 pL.Each OE curve rose at a different rate and was stable after 0.4 millionfor 45% OE, 1.4 million for 30% OE, and approximately 3 million for 15%OE. This demonstrates that after some initial “break-in period”,phosphate stabilized the drop volumes at a high level, giving excellentresults.

Thus, there has been disclosed an additive for reduction or eliminatingkogation in inks used in thermal ink-jet printers. It will be readilyapparent to those skilled in this art that various changes andmodifications of an obvious nature may be made, and all such changes andmodifications are considered to fall within the scope of the inventionas defined by the appended claims.

1. An ink for thermal ink-jet printing comprising a vehicle and ananionic dye, characterized by the presence of at least one oxo aniontherein, said at least one oxo anion selected from the group consistingof phosphates, polyphosphates, phosphate esters, arsenate, molybdate,sulfate, sulfite, and oxalate and present in an amount ranging fromabout 9 mg/L to 14 wt%, wherein said ink further comprises ammonium (NH₄ ⁺) cations, and wherein said at least one oxo anion is selected frommonobasic phosphate (H ₂PO₄ ⁻), dibasic phosphate (HPO ₄ ²⁻), ordiphosphate (P ₂O₇ ⁴⁻) anions or combinations thereof.
 2. The ink ofclaim 1 wherein said vehicle comprises at least one glycol and thebalance water.
 3. The ink of claim 2 wherein said vehicle comprises upto about 10 wt% diethylene glycol and the balance water.
 4. The ink ofclaim 1 wherein said vehicle comprises at least one pyrrolidone and thebalance water.
 5. The ink of claim 4 wherein said vehicle comprises upto about 10 wt% 2-pyrrolidone and the balance water.
 6. The ink of claim1 comprising about 1 to 12 wt% of said dye.
 7. The ink of claim 1wherein said phosphate is in the form of dibasic, monobasic, ordiphosphate anions.
 8. The ink of claim 1 wherein said phosphate esteris selected from the group consisting of mono-organo and di-organophosphate esters.
 9. The ink of claim 1 wherein said at least one oxoanion is present in an amount ranging from about 0.01 to 1 wt%.
 10. Anink for thermal ink-jet printing comprising a vehicle and an anionicdye, characterized by the presence of anions selected from the groupconsisting of phosphates, polyphosphates, and phosphate esters, saidanions present in an amount ranging from about 9 mg/L to 14 wt%, whereinsaid ink further comprises ammonium (NH ₄ ⁺) cations, wherein saidanions are selected from monobasic phosphate (H ₂PO₄ ⁻), dibasicphosphate (HPO ₄ ²⁻), or diphosphate (P ₂O₇ ⁴⁻) anions or combinationsthereof, and wherein the ink provides substantially constant drop volumethrough 5 million resistor firings.
 11. The ink of claim 10 wherein saidvehicle comprises at least one glycol and the balance water.
 12. The inkof claim 11 wherein said vehicle comprises up to about 10 wt% diethyleneglycol and the balance water.
 13. The ink of claim 10 wherein saidvehicle comprises at least one pyrrolidone and the balance water. 14.The ink of claim 13 wherein said vehicle comprises up to about 10 wt%2-pyrrolidone and the balance water.
 15. The ink of claim 10 comprisingabout 1 to 12 wt% of said dye.
 16. The ink of claim 18 wherein saidphosphate anion is in the form of dibasic, monobasic, or diphosphateanions.
 17. The ink of claim 10 wherein said phosphate ester is selectedfrom the group consisting of mono-organo and di-organo phosphate esters.18. The ink of claim 12 wherein said phosphate anion is present in anamount ranging from about 0.01 to 1 wt%.
 19. A method of reducingkogation in an ink used in thermal ink-jet printers, said ink comprisinga vehicle and an anionic dye, characterized in that at least one saltcontaining at least one oxo anion is added thereto, said at least oneoxo anion selected from the group consisting of phosphates,polyphosphates, phosphate esters, arsenate, molybdate, sulfate, sulfite,and oxalate and present in an amount ranging from about 9 mg/L to 14wt%, wherein said ink further comprises ammonium (NH ₄ ⁺) cations, andwherein said at least one oxo anion is selected from monobasic phosphate(H ₂PO₄ ⁻), dibasic phosphate (HPO ₄ ²⁻), or diphosphate (P ₂O₇ ⁴⁻)anions or combinations thereof.
 20. The method of claim 19 wherein saidat least one oxo anion is added in an amount of 0.01 to 1 wt%.
 21. Themethod of claim 19 wherein said phosphate is in the form of dibasic,monobasic, or diphosphate anions.
 22. The method of claim 19 whereinsaid phosphate ester is selected from the group consisting ofmono-organic and di-organic phosphate esters.
 23. A method of reducingkogation in an ink used in thermal ink-jet printers, said ink comprisinga vehicle and an anionic dye, characterized in that from about 9 mg/L to14 wt% of a salt containing an anion selected from the group consistingof phosphate, polyphosphates, and phosphate esters is added thereto,wherein said ink further comprises ammonium (NH ₄ ⁺) cations, andwherein said anion is selected from monobasic phosphate (H ₂PO₄ ⁻),dibasic phosphate (HPO ₄ ²⁻), or diphosphate (P ₂O₇ ⁴⁻) anions orcombinations thereof, and wherein the ink provides substantiallyconstant drop volume through 5 million resistor firings.
 24. The methodof claim 23 wherein said phosphate is in the form of dibasic, monobasic,or diphosphate anions.
 25. The method of claim 23 wherein said phosphateis in the form of dibasic, monobasic, or diphosphate anions.
 26. Themethod of claim 23 wherein said phosphate ester is selected from thegroup consisting of mono-organic and di-organic diphosphate esters. 27.An ink for thermal ink-jet printing comprising a vehicle and an anionicdye, characterized by the presence of at least one oxo anion therein,said at least one oxo anion selected from the group consisting ofphosphates, polyphosphates, phosphate esters, arsenate, molybdate,sulfate, sulfite, and oxalate and present in an amount ranging fromabout 9 mg/L to 14 wt%, wherein said ink further comprises ammonium (NH₄ ⁺) cations.